The intervertebral disks are located between each vertebra and are essential for maintaining the integrity of the spinal column. Deterioration of the intervertebral disks, either by physical damage or through aging, is thought to cause most cases of back pain. In the US alone >50 billion dollars a year are spent on the treatment of back pain and there are presently no cures for a damaged/degenerating intervertebral disk. The center region of the intervertebral disk is composed of a soft, highly hydrated tissue called the nucleus pulposus. Dehydration or damage to the nucleus pulposus can result in inefficient transfer of load between the intervertebral disks, leading to disk herniation and other types of disk disease. The nucleus pulposus is surrounded by the annulus fibrosus, which is thought to help constrain the nucleus pulposus when pressure is applied to the spine. In spite of the essential role the intervertebral disks play in everyday life and the huge financial burden damage to this structure places on our health care system there is very-little known about how this structure forms nor the molecules expressed in this tissue. In this proposal we describe experiments that will uncover the mechanism responsible for transforming the notochord into the nucleus pulposus (Aim 1); determine how the two cell types in the nucleus pulposus are formed (Aim 2); what the role of the somites is in the formation of the intervertebral disks (Aim 3), and identify the molecular pathways responsible for disk formation using previously constructed mouse mutants (Aims 1 and 4) . These experiments will yield a better understanding of the molecular and cellular mechanisms required for formation of the intervertebral disks and identify the progenitor cells responsible for forming this important structure. The identification of the intervertebral disk progenitor populations will allow us in the future to culture cells for use in (stem) cell-based treatment protocols. Public Health Relevance: The intervertebral disks play an essential role in how the spine moves and damage to this tissue causes the majority of the reported cases of back pain. The normal intervertebral disk is composed of three distinct regions: a thick outer ring of fibrous cartilage called the annulus fibrosus; a gel-like material that is surrounded by the annulus fibrosus called the nucleus pulposus; and superior and inferior cartilaginous end plates. The goals of this proposal are to determine the cellular and molecular mechanism(s) responsible for the formation of the nucleus pulposus and to uncover how the two cell types found in this structure, chondrocyte-like and notochordal cells, are derived.